This invention relates generally to the evaluation of liquid-containing porous samples using nuclear magnetic resonance, and according to certain aspects relates to the determination of potential production characteristics of zones of a subterranean formation from which such samples are obtained.
Certain lithological types of formations, such as shaly sandstones, are highly unpredictable in their production of dry hydrocarbons, which are defined herein as hydrocarbons substantially free of water. For example, such formations having high (i.e. 50% or more) total water saturations sometimes produce dry hydrocarbons. Conversely, such formations having lower total water saturations sometimes produce primarily water and little or no hydrocarbons. Therefore, it is necessary for economic production from such formations to determine those zones of a particular formation which will produce dry hydrocarbons.
One technique for making such a determination is actual production testing of zones of the formation. This involves packing a borehole which penetrates the formation at two respective depths so as to seal off a particular zone, and then commencing production from this particular zone. The resulting product is then analyzed for its oil and water content to determine if the product is acceptably "dry". Although this is the most direct and reliable technique for determining the dry hydrocarbon producing potential of a zone of a formation, it is extremely expensive. In offshore operations, in particular, production testing of each zone can cost over $200,000. Therefore, production testing of each zone of interest in a formation can be prohibitively expensive.
Alternatives to production testing include conventional analytical techniques for determining the residual water saturation of a sample or samples from a particular zone. Although defined more specifically below, residual water saturation generally denotes that water which is immobile, and thus not producible from a formation in admixture with hydrocarbons. Once the residual saturation is known, it can be easily deduced whether or not dry hydrocarbons are producible. However, conventional analytical techniques for determining residual saturation, such as the porous plate method involving desaturation of the sample in a pressurized gas atmosphere, can take days or weeks to complete. Such lengthy analysis is neither practical nor economical.